BSA Pre-Unit 4 Speed GearBox

For theoretical Maximum Speed in the Gears download this .xls Worksheet


BSA Pre-Unit 4 Speed Transmission Ratios

Background & History

    The 1st use of the transmission usually referred to generically as the “A10” is in the BB34 Goldstar with duplex cradle frame in 1953, with the remainder of the line following in 1954. These largest BSA machines (“A” Series 500 & 650cc twins and “B” Series 350 & 500cc single-cylinder) were originally offered with many different internal transmission ratio gear-sets to best equip the machines for different purposes such as normal road and moderate performance use, road race, and trials, sidecar and military & police, as shown below. Most of these parts are still available, and can be completely or partially retro-fit into transmission cases, both unit and pre-unit, back to . The exact gear tooth counts and ratios used in these gear-sets have varied slightly over time, and complete interchange may not be possible. Some of these parts were probably used in the older M20, M21, B31 &c. transmissions.
    My purpose is to supply information about what choices are available based on combinations of factory parts, explain some of the differences between the various ratios, describe in a general way what needs to be done for a selection, and warn you of potential difficulties. I will limit my comments to methods that have either already been used successfully, or that I believe are sound concepts that are worthy of your time and effort.
Ratio Calculations
     BSA 4 speed transmission’s intermediate gears (1st through 3rd) are the result of power transmission through two pairs of mating gears, one of which is the mainshaft high gear being turned by the layshaft (countershaft) high gear. The gear ratio is determined by taking the product of multiplying the two pair's tooth count ratios together: the high gear ratio times the ratio of the individual gear pair selected (1st, 2nd or 3rd).
    Please note that the total tooth count of each pair of mating gears (mainshaft and layshaft) is 43: 26/17, 25/18, etc. This is fixed by the tooth form's pitch diameter and the center-to-center distance of each pair (which is obviously the distance between the shafts) is a constant. This is a useful safety check to see if the assembly you have is really a useable gear set, or merely a random assortment of parts.
    Under certain conditions, a mating pair (with the 43 total) may be substituted for another ratio: 2nd for 3rd, etc. but this must be researched on an empirical basis and may require minor modifications to the length, dog shape and size, etc.
    The high gear ratio is the ratio of the tooth counts of the high gear pair for the transmission type selected (see “High Gear Pair” in Column 4 below). For example, the STD. transmission (the most common model) has 27 teeth on the mainshaft high gear and 16 teeth on the layshaft gear, so the high gear multiplier (Column 5) will be 1.68750 for this transmission.
    The 2nd number is the ratio of the tooth counts of the pair of gears (mainshaft & layshaft) for that ratio: 1st, 2nd or 3rd.
    The std. transmission has 27 teeth on the mainshaft 1st gear and 16 teeth on the layshaft 1st gear, so the gear ratio for the 1st gear pair (Column 8) is 1.6875 for 1st gear in the std. transmission. To get the actual effective ratio of 1st gear, multiply those two numbers together:

High Gear Multiplier

×

1st Pair Ratio

=

1st Gear Ratio

1.54941

×

1.68750

=

2.58088-1

    In 4th gear the mainshaft high gear is locked to the mainshaft, so the ratio is 1-1. No power is transmitted through the intermediate gears. Although they still turn, they’re only idling.
    This Table shows some common transmissions, their internal gear-sets, and how the individual ratios are generated.
    For transmissions based on the Std (standard) high-gear pair, the high gear tooth counts are: 26 for the mainshaft gear and 17 for the layshaft gear, so the 1st number is always 1.54941 for transmission gear-sets based on the “std.” high gear pair.
    For transmissions based on the Close high-gear pair, the high gear tooth counts are: 25 for the mainshaft gear and 18 for the layshaft gear; the 1st number is always 1.38889 for transmission gear-sets based on the “close” high gear pair. 
     The overall governing component for all ratio selection is the high gear pair, which determines the range of ratio spread between 1st gear and 4th gear.
    The two original factory gear-sets are shown in color for clarity: “Std.” ratios in blue, and “Close” ratios in green.

Table 1: Internal Gearbox Ratios & Gear Tooth Counts

Gear

Ratio Type & Case Marking

Actual Ratio

High Gear Pair

Multiply By

L/S Gear

M/S Gear

Pair Ratio

1st

RR.T2

1.7545-1

25/18

1.38889

24

19

1.2632

RR., RR.T

1.9290-1

25/18

1.38889

25

18

1.3889

DAY., DAY.T

2.1242-1

26/17

1.54941

25

18

1.3889

SC., SC.T

2.3438-1

25/18

1.38889

27

16

1.6875

STD., STD.T

2.5809-1

26/17

1.54941

27

16

1.6875

ARRT., ASC., ACS.T, STD.2

2.8770-1

25/18

1.38889

29

14

2.0714

TRI., TRI.T

3.1680-1

26/17

1.54941

29

14

2.0714

Alternate 1st #1

1.9319-1

26/17

1.54941

24

19

1.2632

2nd

RR.T2

1.3257-1

25/18

1.38889

21

22

0.9545

RR., RR.T

1.3257-1

25/18

1.38889

21

22

0.9545

DAY., DAY.T

1.4598-1

26/17

1.54941

21

22

0.9545

SC., SC.T

1.7545-1

25/18

1.38889

24

19

1.2632

STD., STD.T

1.7588-1

26/17

1.54941

23

20

1.1500

ARRT., ASC., ACS.T, STD.2

1.7545-1

25/18

1.38889

24

19

1.2632

TRI., TRI.T

2.3391-1

26/17

1.54941

26

17

1.5494

Alternate 2nd #2

1.9319-1

26/17

1.54941

24

19

1.2632

Alternate 2nd #3

1.5972-1

25/18

1.38889

23

20

1.1500

3rd

RR.T2

1.0996-1

25/18

1.38889

19

24

0.7917

RR., RR.T

1.0996-1

25/18

1.38889

19

24

0.7917

DAY., DAY.T

1.1012-1

26/17

1.54941

18

25

0.7200

SC., SC.T

1.3257-1

25/18

1.38889

21

22

0.9545

STD., STD.T

1.2108-1

26/17

1.54941

19

24

0.7917

ARRT., ASC., ACS.T, STD.2

1.3257-1

25/18

1.38889

21

22

0.9545

TRI., TRI.T

1.4598-1

26/17

1.54941

21

22

0.9545

Alternate 3rd #4

1.0000-1

25/18 1.38889

18

25

0.7200

4th

RR.T2

1.0000-1

25/18

1.38889

NA

RR., RR.T

1.0000-1

25/18

1.38889

DAY., DAY.T

1.0000-1

26/17

1.54941

SC., SC.T

1.0000-1

25/18

1.38889

STD., STD.T

1.0000-1

26/17

1.54941

ARRT., ASC., ACS.T, STD.2

1.0000-1

25/18

1.38889

TRI., TRI.T

1.0000-1

26/17

1.54941

    Note that not all possible gear pair combinations were used in the original ratios shown in Table 8. I have inserted the other four theoretical choices, which may be possible (although I have no experience or information). Not all of them have any practical use.
    Alternate 1st gear #1 (1.9319-1) creates an extremely close ratio, approximately the same ratio as the close ratio “RR., RR.T” (1.9290-1) and only suitable for road racing or LSR use. However, it allows this close ratio to be inserted in transmissions fitted with the std. ratio (26/17) high gear pair such as “DAY., DAY.T ”.
    Alternate 2nd gear #2 (1.9319-1) creates a std. ratio in between the fairly wide “STD., STD.T” (1.7588-1) and the extremely wide “TRI., TRI.T ” (2.3391-1) ratios in these std. ratio transmissions.
    Alternate 2nd gear #3 (1.5972-1) creates a close ratio in between the fairly wide “SC., SC.T” and “ARRT., ASC., ACS.T, STD.2” (1.7545-1) and “RR.T2” and “RR., RR.T ” (1.3257-1) ratios in these close ratio transmissions.
    Alternate 3rd gear #4 (1.0000-1) has no practical purpose.








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